Neutron-diffraction study of the metamagnetic phases in HoNi2B2C

Neutron diffraction has been used to determine the magnetic phase diagram of HoNi2B2C, in the plane of temperature and magnetic field up to 4.6 T, for two different crystallographic orientations relative to the applied magnetic field. For both orientations two first-order metamagnetic transitions are observed below 5.5 K as the applied field is increased. In both cases the first transition is to a state in which the magnetic structure is modulated along the c axis with wave vector delta=[0,0,0.667]. With the field applied parallel to the [110] direction the second transition is to a saturated paramagnetic state in which the spins are aligned ferromagnetically by the applied magnetic field. With the field parallel to the [0 (1) over bar 0] direction, the second transition is into a state in which the magnetic structure is modulated along the b axis with wave vector delta= [0,0.610,0], in disagreement with a simplified theoretical model.